Structural Biology

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The frog Xenopus tropicalis

A range of evidence, including whole genome sequencing, shows that X. tropicalis has a relatively simple genomic structure, suggesting a consistently diploid genetic history. Unlike zebrafish and other Xenopus species, its genomic organisation seems not to have diverged significantly from that of most vertebrates, and genetic and antisense strategies are likely to reveal novel functions. Its relatively short lifecycle facilitates multigeneration transgenic approaches, and adapting embryological and molecular techniques and probes from X. laevis to the smaller X. tropicalis embryo is usually straightforward. Recessive phenotypes may be readily uncovered, stable transgenic lines can be bred more quickly, and pseudogenes or alleles which complicate promoter analysis and gene knockdowns are less commonly encountered.

Species

X. laevis

X. tropicalis

ploidy

allotetraploid

diploid

N

18 chromosomes

10 chromosomes

genome size

3.1 x 109 bp

1.7 x 109 bp

temp. optima

16-22o C

25-28o C

adult size

10 cm

4-5 cm

egg size

1-1.3 mm

0.7-0.8 mm

eggs/spawn

300-1000

1000-3000

generation time

1-2 years

<4 months

The Genus Xenopus

X. tropicalis inhabits small bodies of water in equatorial lowland west Africa. Xenopus systematics remains an active field, and it is possible that additional species will be described. The genus forms a polyploid series, with species having 20 (tropicalis), 40 (epitropicalis), 36 (laevis and others), 72, and 108 chromosomes. X. tropicalis is currently considered the lone diploid member of the genus. It has at least one close tetraploid relative, X. epitropicalis, from which it is difficult to distinguish morphologically. Since the ranges of tropicalis and epitropicalis (as well as other Xenopus species) overlap extensively in the wild (tropicalis tending northward and westward from Cameroon, and epitropicalis more prevalent south and east), it is appropriate to be cautious about species identity when purchasing wild-caught frogs (see karyotyping protocol).

Nomenclature: Xenopus vs. Silurana

X. tropicalis and epitropicalis form a subgroup within the genus Xenopus, and it has been debated whether they should be placed in a separate genus (Silurana). Briefly, some morphometric analyses suggested that the tropicalis/epitropicalis group resembles other Xenopusless than it does other pipids (and hence that they should be placed in their own genus), while ribosomal DNA sequence analysis is consistent with a closer relationship within Xenopus.

X. laevis vs. X. tropicalis

Xenopus (Silurana) tropicalis is a close relative of X. laevis (viable hybrids between the two species have been reported), and shares virtually all of X. laevis' advantages as an embryological system. In addition, it features a much shorter generation time (as little as three months to sexual maturity), and a smaller diploid genome (twenty chromosomes, with about 1.7 x 109 base pairs, versus thirty-six chromosomes and 3.1 x 109 bp for X. laevis). Adult X. tropicalis, at 4-5 cm, are considerably smaller than the 10 cm X. laevis, and consequently can be housed more efficiently; eggs are also somewhat smaller (0.6-0.7 mm vs 1-1.3 for X. laevis), but still amenable to manipulation, and are more abundant (up to 9000 per spawning versus 300-1000). The ancestors of X. laevis and tropicalis are thought to have diverged about 90 million years ago; laevis’ genome duplication is thought to date from ∼30 million years ago. The genome size and number of chromosomes compare favorably to those of mouse (forty chromosomes, 3 x 109 bp) and zebrafish (fifty chromosomes, 1.6 x 109 bp).

X. tropicalis in the laboratory

The embryological techniques and molecular assays which have been described for X. laevis are readily adapted to X. tropicalis, but may be supported by multigeneration genetic analyses and genomic resources. X. tropicalis' diploid genome facilitates uncovering recessive phenotypes, and pseudogenes are far less likely to complicate promoter analysis than in the tetraploid X. laevis. Its genomic sequence is undergoing assembly, and large-scale EST sequencing projects are also underway. Using mutants or transgenic animals in highly-developed tissue transplantation regimes will facilitate analysis of individual animals containing tissues of more than one genotype. Such genetic mosaic analyses have been very useful in studies of Drosophila embryogenesis, but are technically challenging in extant vertebrate models.